While en route to Pacifica for a mission, Lt Geordi Laforge is telling Lt Commander Data an amusing joke. After Geordi delivers the punchline, Data pauses for a moment to consider it, before analysing its content and determining why the joke is, in fact, amusing. Hysterical, in fact. Suddenly, Data’s face contorts slightly before emitting a burst of forced laughter. Data immediately returns his attention to his station, whilst Commander Riker and Counsellor Troi chuckle at what they just witnessed.

After enquiring how long it will take to get to Pacifica, Riker orders that the speed be increased to Warp 6, to which Laforge replies, “aye sir, full impulse”.

The crew then discuss what they’ll do if they get there early, with Troi looking forward to a nice swim. Data points out that it’s possible to create an environment for swimming in the Holodeck, but Troi says that there’s nothing else like moonlight swimming… leaving Data confused as to how you can swim in moonlight. Worf is also not keen on the idea, likening swimming to bathing.

Jean-Luc Picard receives a code 47 emergency message from Walker Keel, an old friend who is captain of the Ambassador-class USS Horatio.

Keel asks Picard for an immediate secret rendezvous on the abandoned mining colony on Dytallix B. Even though it is a secured channel, Keel insists they meet face to face. Something is beginning.

Act One

Picard quickly arrives on the bridge and asks Data if he knows Dytallix B. He orders the helm to travel there immediately, with no record of the change and no communication to Pacifica. He goes to his ready room, leaving Riker to wonder what is going on.

Riker asks Data to look for information on Dytallix B. It’s one of the planets circling the star, Mira. One side of the planet always faces the sun (similar to Earth’s Moon), where temperatures can reach 180 degrees. There are mines situated around the temperate zone between the two sides of the planet, but they have long since been deserted. With no indigenous life, it’s nothing but a hunk of rock… and a number of other similar descriptions that Data started to spew forth until Riker said that he understood.

As they approach the planet, they detect three other ships already in orbit… two frigates and an Ambassador-class cruiser, the USS Horatio, Walker Keel’s ship. There are also three lifeforms on the planet, all gathered inside the entrance to a mining tunnel.

Picard orders that the coordinates to that location be transferred to the transporter, and he heads there on his own… much to Riker’s confusion.

On beaming down to Dytallix B, Picard moves towards the entrance of the mining tunnel, where he meets up with Walker and two other captains, Tryla Scott and Rixx. Walker, after confirming Picard’s identity by attempting to trip him up with a series of questions about his past, introduces Picard to the others – Starfleet’s finest, as Picard puts it. Walker tells him about their suspicions of a conspiracy of some sort reaching up to the highest levels of Starfleet Command. They don’t have all the answers, but they cite recent suspicious orders and deaths. They are convinced that people, even top-level personnel, are “changing” somehow, and suggests that the Enterprise might be targeted soon. Keel already suspects his first officer and chief medical officer. The only sign of the change appears to be loss of long-term memories. They don’t have all the answers, but they ask that Picard stay vigilant. This meeting never happened.

Act Two

After returning to the Enterprise-D, he first consults Deanna Troi, saying he believes Keel and trusts him as a friend, even though he is violating Starfleet regulations.

Troi is concerned that Picard is putting himself at risk by enteraining Walker’s theory, by essentially making him complicit in the violation. He is, however, going to keep this from the rest of the Enterprise crew, so as not to implicate them.

They resume course to Pacifica at Warp 8, even though it still puts them almost 10 hours behind their original schedule.

Picard has an assignment for Data, one that is specifically suited to his talents.

He asks Data to review Starfleet directives of the past six months. While Data is reviewing the records, the USS Enterprise-D continues on to Pacifica, but along the way encounters an unusual disturbance in a nearby sector. Picard re-directs the Enterprise toward it, and they eventually identify it as shipwreck debris. Speculating that it could be one of the three ships they encountered at Dytallix B, Worf informs them that based on the volume of debris, it could only be the USS Horatio, Walker Keel’s ship.

Act Three

In light of Keel’s death, Picard tells Riker about the suspicions Keel voiced in the observation lounge and connecting them with Rear Admiral Quinn‘s earlier that year, but Riker isn’t convinced by the conspiracy theory.

Meanwhile, Data is still scanning through the results of his assignment, remarking to himself how extraordinary they are. Suddenly, the computer interrupts asking for clarity of his request. Data points out that he wasn’t making a request, but simply talking to himself. He takes this as quite a revelation, marvelling out loud how this is a form of human idiosyncracy, triggered by fascination… or senility. Data goes on explaining, until the computer interrupts him with the words, “Thank you, sir. I comprehend.” Data appears almost embarrassed by this – what I can only describe as a – smack-down, and asks the computer to continue scanning records.

Data goes to the observation lounge, where Picard and Riker are still discussing the destruction of the Horatio, and whether it was sabotage. Data reports what he has found: during the past six months there has been a great deal of “uncustomary reshuffling of personnel – usually in the command areas” of a select number of starbases. Furthermore, the new officers have had a great deal of contact with the highest levels of command. When Riker asks why this wasn’t discovered before, Data suggests that Starfleet’s left hand didn’t know what its right hand was doing. Data hypothesizes that the reorganizations are an attempt by a hostile force or individual to control important sectors of Federation territory. Riker suggests that this might be the prelude to an invasion, but can’t even begin to think who might be behind it. Picard believes that this could be a threat to the entire Federation, Picard orders the Enterprise-D to return to Earth.

Upon entering orbit, the Enterprise-D is contacted by three admirals from Starfleet Command – Savar, Aaron, and Quinn – requesting an explanation for their return. Picard states that he would prefer a discussion of that sort occur in private; the three admirals pause their transmission to convene.

Picard, Riker, Troi, and Laforge discuss the situation, each saying that they aren’t comfortable with it. Just as Worf is saying that he doesn’t trust them, the transmission resumes, and Admiral Savar invites Picard and Riker to dinner at Starfleet Headquarters for the discussion to take place. Quinn says that he will not be able to attend the dinner, but that he would like to see the Enterprise-D again. The transmission ends.

In the transporter room on Earth, Quinn looks at a scorpion-like creature he has in a case. Remmick (remember him?) enters the room, and beams Quinn up to Enterprise.

Act Four

When Quinn is on board the Enterprise-D, he bluffs his way through references to old times, stating that his earlier perception of a threat to the Federation was merely a metaphor for the “tumultuous process” of assimilating new species into the Federation. Picard realizes that Admiral Quinn is an impostor of some sort and tells Riker to observe Quinn closely while he beams down alone to dinner. Also, he wants Doctor Crusher to give him a medical examination under false pretences; after Riker reaches his conclusions, he is to join Picard on Earth, in force if necessary. Picard then beams down and meets with Savar and Aaron, as well as Remmick. Picard comments that the place is usually busier than it appears now, Remmick says that it’s “a quiet night”, echoed by Savar.

On the ship in the guest quarters, Quinn offers to show Riker the creature and tells him about it. It was discovered accidentally by a Starfleet survey team on an uncharted planet, and Quinn refers to it as “a superior form of life”. He intended to show it to Dr Crusher, but when Riker says he’ll get his science officer to examine it, Quinn grabs his arm very strongly, saying the creature will only like Riker. After a brief scuffle, in which Quinn displays amazing strength, Riker is knocked unconscious, but not before calling security.

On Earth, Picard, Savar, and Aaron raise a toast to the USS Horatio… but then Picard says that it’s actually something that he wanted to discuss. When he asked what they suppose happened to the ship, Aaron suggests that it was due to the negligence of her captain.

In Quinn’s guest quarters on the Enterprise, Worf and La Forge arrive, and declare a medical emergency. Quinn says that Riker slipped and hit his head, and then announces his departure. When they try to detain him, however, he throws La Forge out into the corridor, his body knocking down the door. He turns to Worf, who slowly rises.

Act Five

He similarly beats Worf before being repeatedly phasered with increasingly powerful stun settings into unconsciousness by Dr. Crusher. In sickbay, Crusher’s scans show that Quinn really is Quinn, but she discovers a bizarre appendage sticking out of the back of his neck.

On Earth, the two admirals try to subtly persuade Picard that there is in fact no conspiracy.

Dinner is announced, so Picard takes a moment to contact Riker. Crusher answers on Riker’s combadge, as Riker is unconscious. Crusher informs Picard that a parasitic creature of some sort has taken control of Quinn and all his brain functions. The spike at the back of Quinn’s neck appears to be a gill, which is a helpful indicator of those who have been taken over by one of the creatures. Crusher states her doubts about removing the creature, believing it would kill Quinn. She instructs Picard to set his phaser to kill rather than stun, which has little effect on the creature or its host; Picard, however, believing that no one beams down to Starfleet Headquarters armed, has no phaser with him at all.

Picard goes in to dinner. The dish being served is a bowl filled with worms. Picard is disgusted, and realizes that everyone at the dinner has been infiltrated by the aliens.

Picard gets up to leave, and runs into Riker. Riker has also apparently been taken over by the creature: he has a gill in his neck. So, too, has Captain Scott. The “conspirators” announce that they have known of Picard’s intentions the whole time. They talk of their plan to infiltrate the Enterprise-D. Riker moves to eat, but instead he pulls out a type 1 phaser and shoots the dining security guard. When Captain Scott draws her phaser at Riker, Picard grabs at her arm and throws off her aim, allowing Riker to shoot her as well. As she hits the floor, a scorpion-like creature crawls out of her mouth and scuttles away. Admiral Savar begins to subdue Riker with a Vulcan neck pinch.

Picard then grabs the Captain Scott’s phaser and uses it to take down Savar. Admiral Aaron flees the dining room, and Picard and Riker pursue and shoot him in the hallway after he returns fire. The admiral falls to the ground, and the parasite infesting him crawls out of his mouth and goes under a nearby door.

They follow it to find Remmick sitting in a chair. He turns to look at it, unconcerned; Riker goes to shoot it, but Picard stops him, as the creature crawls up Remmick and enters his mouth. He swallows it, and it begins moving around in his neck. “We mean you no harm“, he says as he stands, his neck noisily bulging in and out. “We seek peaceful coexistence“.

Picard and Riker open fire with their phasers, knocking Remmick back into the chair. Riker aims higher and destroys Remmick’s head, the outer layer of skin vaporizing and the rest exploding. Remmick’s chest cavity begins to dissolve and a large creature breaks through with several blood soaked parasites trickling behind, screaming at the two Starfleet officers. With a disgusted look on his face, Picard opens fire again with Riker following suit, both phasers continuing until the creature is vaporized, leaving several blood soaked parasites around the chair and floor, and the decapitated smoldering remains of what was once Lieutenant Commander Remmick.

Back aboard the Enterprise-D, Picard notes in his log that Admiral Quinn will make a full recovery, and with the death of the “mother creature” inside of Remmick, the remaining parasites died while Riker reveals that Dr. Crusher had simulated the gill on the back of his neck in order to fool everyone, including Picard. As the Enterprise-D moves on into space, Data theorizes that the message Remmick was transmitting at the time he died was a homing beacon.

As the Enterprise moves on through space, the sound of electronic beeping is heard, focused on an unexplored patch of the galaxy, leaving the ominous possibility that some or all of Remmick’s message might still reach its intended recipients, letting the parasites know where to find Earth.